Heat engines are used to convert heat or thermal energy into useful mechanical work and are often used in power generation plants. One example of a heat engine is an expander-generator system which generally includes an expander (e.g., a turbine) rotatably coupled to a generator or other power generating device via a common shaft. As working fluids are expanded in the expander, the shaft connecting the turbine and generator rotates and generates electricity in the generator.
Most power plant expander-generators are based on the Rankine cycle and obtain high temperature and pressure working fluids through the combustion of coal, natural gas, oil, and/or nuclear fission. The typical working fluid for Rankine cycles is water (steam). Recently, however, due to perceived benefits in terms of hardware compactness, efficiency, and heat transfer characteristics, there has been considerable interest in using super-critical carbon dioxide (ScCO2) as a working fluid for certain expander-generator applications. Notable among such applications are nuclear, solar, geothermal, and waste heat energy conversion cycles.
Although ScCO2 has several remarkable advantages as a working fluid, its low specific heat makes its use in waste-heat recovery cycles problematic. For instance, its low specific heat results in a relatively small temperature drop in the ScCO2 gas through a typical heat cycle pressure/expansion process. This relatively small temperature drop can limit the amount of recoverable waste heat energy using simple Rankine (or Brayton) cycles, especially for applications utilizing waste heat streams having high initial gas temperatures.
In an effort to recover more waste heat energy in a heat recovery cycle, one or more recuperators are typically added to the system. The recuperators transfer a portion of the heat energy remaining in the working fluid after expansion to the working fluid at some other place in the thermodynamic cycle (usually after the pump or compression process, depending on the type of cycle being used). Using multiple recuperators increases the structural footprint of the system since each individual piece of equipment in the system is normally mounted separately to a large structural baseplate. Accordingly, although adding conventional recuperators to the system serves an important purpose, it simultaneously consumes valuable floor space that could otherwise be used for other beneficial applications or reduce the size and cost of the system.
What is needed, therefore, is a simple, compact waste heat recovery system that utilizes recuperators but also reduces the overall footprint and cost of the system.